Coal is a major source of air pollution, mining accidents, and environmental damage. Unfortunately, we can't live without it.

The coal question remains perhaps the largest and most difficult issue in the clean-tech and energy world. Proponents of solar, wind, and even nuclear power tout themselves as cleaner and safer alternatives. Environmental activists and many scientists also warn that "clean coal" technologies will only dupe the public into a false sense of security.

On the other hand, coal use continues to climb, particularly in China. Clean coal technologies, along with carbon capture and sequestration, may be the only practical way to adapt to climate change. The profits, moreover, are potentially massive.

"Clean coal is the biggest opportunity" in clean tech, said Stephan Dolezalek, a partner at VantagePoint Venture Partners earlier this year. "If you can solve that problem, it will be bigger than Google."

What are those opportunities? They are mostly on the drawing board now. Here's a primer on the basics of coal:

Q: How much coal is there?
Approximately 998 billion tons of recoverable coal sits underground, according to a 2006 estimate from the International Energy Agency. The U.S. has the most, with 268 billion tons, followed by Russia (173 billion tons), China (126 billion tons) and India (102 billion tons). The four collectively hold 67 percent of the recoverable reserves.

In 2006, 1,438 U.S. mines produced 1.163 billion short tons of coal, according to the U.S. Department of Energy, a 2.8 percent increase from the year before. A short ton is 2,000 pounds.

A ton of coal, depending on the grade, has as much heat energy (25 million BTUs) as 4.5 barrels of oil (PDF). There are probably only left for human consumption, and not all of it can be recovered. Thus, there's more than twice as much coal out there than oil.

How fast is demand growing?
Steadily, but ominously. Coal accounted for 26 percent of energy consumed in 2004 worldwide, according to the U.S. Energy Information Agency, and will grow to 28 percent by 2030. Total energy consumption, however, will be going up a few percentage points a year, so in that same period of time, coal consumption will rise a whopping 74 percent, form 114.4 quadrillion BTUs to 199 quadrillion BTUs.

India and China will account for 72 percent of the increase, but coal consumption is expected to also rise in Russia, South Africa, and the U.S. The U.S. is something of a wild card. With carbon taxes and more alternative energy, the growth could decline, but coal will still be a big part of the energy profile.

"Ninety percent of the fossil fuel reserves in the U.S., India, and China are in coal, and China and India are not going to move from this fuel in the future," said Jeremy Carl, a research fellow in the program for Energy and Sustainable Development at Stanford University. "They are not going to turn off the lights."

China last year erected 90 gigawatts' worth of coal plants last year alone, Carl noted. That's bigger than the electrical grid of the U.K.

Where does it get used?
Primarily in electrical power plants. In the United States, roughly 1.03 billion tons of the 1.1 billion tons of coal consumed (PDF) in 2006 got gobbled up by power plants. Coal accounted for 49 percent of the electricity generated in the U.S. in 2006, a slight decline from 2005 due in part to warmer temperatures. (Nuclear power was second, with about 20.2 percent, while natural gas clocked in at 18.8 percent. Solar and wind barely account for 2.4 percent.)

How does coal affect pollution?
Coal accounted for 39 percent of carbon dioxide emissions in 2004 (behind oil) but is expected to pass oil for the No. 1 spot in 2010, according to the EIA. Even if the United States were to replace every incandescent bulb in the country with compact fluorescents, the benefits would be eradicated by the carbon dioxide from two coal-fired plants over a year, said Ed Mazria, founder of Architecture 2030. The nonprofit encourages builders, suppliers, and architects to move toward making carbon neutral buildings by 2030.

"The only fossil fuel that can fuel global warming is coal. If you stop coal, you stop global warming. End of story," Mazria said.

Other pollutants include nitrogen compounds, sulfur, aluminum, silicon, and even trace amounts of radioactive materials like uranium. China has banned the use of coal burners in homes in cities like Beijing, but coal pollution remains a large health hazard in the country.

Environmental and health problems include acid rain, polluted water systems, stripped forests, and mining hazards. Deaths attributed to coal range from several hundred to several thousand a year, depending on who does the counting and which respiratory deaths get attributed to coal.

How much does it cost?
In the early '70s, natural gas was a cheaper source for generating electricity, but coal surpassed it in 1976 and has been at the bottom ever since. In 2005, generating a million BTUs from coal cost $1.54, compared with $8.20 for natural gas. Coal prices are rising, but so is the cost of everything else. Solar thermal plants, which generate electricity with heat from the sun, are approaching the cost of natural gas plants.